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INTEGRAL measures the hard X-ray spectrum of the Be/X-ray binary XTE J1859+083

ATel #7425; D. Malyshev, C. Ferrigno (ISDC/University of Geneva, Switzerland) D. Gotz (CEA/Saclay, France)
on 21 Apr 2015; 12:22 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

Subjects: X-ray, Neutron Star, Transient, Pulsar

During the INTEGRAL observations performed from 2015 April 17, 19:47 to April 19, 20:01 UTC, the IBIS/ISGRI instrument detected a highly significant signal from a transient source, positionally coincident with the Be/X-ray pulsar XTE J1859+083 (ra, dec) = (284.78 ; 8.25) , which is reported to be in outburst since 2015-02-08 (ATeL #7034).

The source is detected in the IBIS/ISGRI mosaic images with fluxes 62 ± 2 mCrab in the 20-40 keV energy band and 40 ± 2 mCrab in 40-80 keV band (effective exposure of 39 ks). During these observations, XTE J1859+083 was outside of the lower energy range JEM-X instrument field of view.

To obtain a broad-band estimate of the source spectrum, we have extracted the average Swift/XRT spectrum from the public archive using the Leicester online tool (http://www.swift.ac.uk/user_objects/). In the archive, there are 6.2 ks of data available in window-timing mode, which is not affected by pile-up at the source flux level. The observations are taken in several snapshots between 2015-03-12 and 2015-04-08 and the spectrum is grouped to have at least 30 counts per bin.

We fitted the averaged Swift/XRT and IBIS/ISGRI spectra using an absorbed cut-of power law and inter-calibration constant to account for the source variability. The fit (χ2red. = 0.9 for 636 d.o.f.) yields NH=2.19±0.07 × 1022 cm-2, Γ=1.25±0.05, and Ecut=26±4 keV. The 1-10 and 20-100 keV absorbed fluxes are 6.0×10-10 and 8.5 ×10-10 erg/cm2/s, respectively. We caveat that XTE J1859+083 is variable and these spectral properties should be considered as a a rough estimate of the average source properties, which are consistent with its proposed high mass X-ray binary nature.